Stand-by instruments are stand-alone onboard instruments which generate and display flight information which is essential to the pilot of an aircraft in the event of failure of primary onboard instruments. This flight information, generally obtained with less precision than that of the primary onboard instruments, essentially relates to the speed, altitude and attitude of the aircraft. In order to guarantee the stand-alone capability of the stand-by instruments in relation to the primary onboard instruments, the stand-by instruments must have their own sensors in order to generate and display the speed, altitude and attitude of the aircraft. In particular, the stand-by instruments normally comprise a static pressure sensor, a total pressure sensor and an inertial unit.
The static and total pressure sensors are connected respectively to a static pressure connector and a total pressure connector located on the skin of the aircraft. The static pressure allows the altitude of the aircraft to be determined. The difference between the total pressure and the static pressure allows the speed of the aircraft to be determined in relation to the air.
The inertial unit comprises 3 gyrometers and 2 or 3 accelerometers. The gyrometers measure the speed of rotation of the sensor referential, here a system of axes linked to the stand-by instrument, in relation to an inertial referential. Through integration of the rotation speeds, it is possible to identify the position of the stand-by instrument in relation to the inertial referential and therefore, knowing the position of the stand-by instrument in relation to the aircraft and the position of the local geographical frame of reference in relation to the inertial frame of reference, it is possible to identify the position of the aircraft in relation to the local geographical frame of reference. The position of the aircraft in relation to the local geographical frame of reference, referred to as the attitude of the aircraft, is determined in relation to a roll axis, a pitch axis and a yaw axis, and the movements around these axes are referred to respectively as the roll, pitch and yaw. Accelerometers measure non-gravitational forces applied to the aircraft, from which translation accelerations of the sensor referential in relation to the inertial referential are deduced. The combination of gyrometers and accelerometers enables a precise determination of the attitude of the aircraft, the data supplied by the accelerometers being used in preference to the data supplied by the gyrometers in the static or quasi-static flight phases, and the data supplied by the gyrometers being used in preference to the data supplied by the accelerometers during the dynamic phases of the flight.
When an aircraft, and in particular a stand-by instrument, is powered up, the inertial unit of the stand-by instrument must be initialized in order to supply the most reliable attitude information possible during the flight. This initialization includes an alignment phase, consisting notably in estimating the drift of the different gyrometers, i.e. the speed of rotation measured by the gyrometers in the absence of any movement of the latter. The gyrometers being electronic sensors, their drift may differ between two different power-ups of the inertial unit, to the point of rendering unusable any measurement carried out by these gyrometers and therefore any attitude displayed by the stand-by instrument. It is therefore necessary to determine the drift of the gyrometers on every power-up. Moreover, the alignment of the gyrometers must be carried out in the absence of any movement of the inertial unit, otherwise a movement of the inertial unit will be integrated into the drift of a gyrometer.
To ensure the correct alignment of the gyrometers of an inertial unit, it is known to check for the presence or absence of movements of the inertial unit by means of the accelerometers of the inertial unit. For the entire duration of the alignment, the accelerometers measure the non-gravitational forces of the inertial unit in relation to the inertial referential. In the event of movement of the inertial unit during the alignment, measured by the accelerometers, the stand-by instrument, at the end of the alignment, invalidates the determination of the drift of each gyrometer, displays a message indicating the detection of movement to the pilot and asks the pilot to restart the alignment either by switching off the stand-by instrument then powering it up again, or by pressing a button on the front surface of the stand-by instrument. This restart of the alignment is imperative insofar as the availability of the stand-by instrument, and therefore the alignment of the inertial unit, is a necessary condition for the aircraft take-off authorization.
A solution of this type presents a plurality of disadvantages. A first disadvantage is the wait for the end of the alignment in order to indicate the detection of a movement during the alignment. It is therefore only at the end of the alignment of the gyrometers that the pilot is aware of the invalidation of the alignment and can restart it. Consequently, the time elapsed between the detection of movement and the end of the alignment is lost. A second disadvantage is the loss of the estimation of the drifts carried out between the start of the alignment and the detection of a movement. At the end of the invalidated alignment, the entire alignment procedure is restarted, entailing the risk that the estimated drift has been distorted by the movement. Moreover, if the alignment is restarted by a hardware reset, i.e. by switching off the stand-by instrument then powering it up again, there is a risk that the drift of the gyrometers will change, rendering the preceding determination of the drifts obsolete. A third disadvantage is the impossibility, in certain situations, of being able to carry out an alignment. This may notably occur if the aircraft has started up on a moving platform. In most cases, the movement of the platform, for example due to the swell of the sea, cannot be prevented. The aircraft must then wait for the cessation of the movements, in this case a calming of the swell, to be able to take off. An immobilization of this type is indisputably detrimental to the economic efficiency of the aircraft.